Implement the HostGetSupportedImportAssertions, whose purpose
is to filter the list of import assertions exposed to the embedder to
only those assertion with keys that the embedder recognizes. See
https://tc39.es/proposal-import-assertions/#sec-hostgetsupportedimportassertions.

This change doesn't actually implement it as a callback, but instead
passes the supported assertions during creation of the Isolate via
CreateParams. This expresses clearly the requirement that the supported
assertions must never change for the lifetime of the Isolate.

Note that we still need to maintain all assertions in a map
while parsing the import assertions clause, because duplicate keys for
an unsupported assertion still needs to be detected as a parse error. So,
the filtering is done later during
SourceTextModuleDescriptor::AstModuleRequest::Serialize.

The actual filtering algorithm simply iterates the assertions and the
supported assertion keys in a nested loop. There's currently only one
assertion in use ("type"), so there should be no reason to get too
clever here unless at least several more assertions are generally
supported.

Bug: v8:10958
Change-Id: I9a2d965e9d452718d0ddfe9dca55b7b4ed963019
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2572173Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#71776}